PROJECT 4 : Immune Tolerance and Stem Cell Transplantation Impaired immune responses are common in cancer and a particular feature of chronic lymphocyfic leukemia (CLL). The immune dyfuction in CLL is characterised by hypogammaglobulinemia and autoimmune phenomena and infecfious complicafions are a major cause of morbidity and mortality in this disease. Previous work in this Program has demonstrated a direct effect of CLL cells on T cells, both in human samples and in the Eji-TCL1 transgenic mouse model of this disease, that result in changes in acfin polymerizafion in T and NK cells and failure ofthese cells to mount effecfive immune synapses with antigen presenting cells. The central hypothesis of this project Is that specific T cell defects result from interaction of CLL cells with the patient's immune system and that repair of these defects will be required to maximize T cell mediated immune responses in vivo. We therefore seek to characterize the basis for defective immune cell function in CLL and repair these defects for future therapeutic intervenfion. We shall examine this in human samples from pafients with CLL and in an E(i-TCL1 transgenic mouse model of this disease. Since most agents that are used to treat CLL also add to the immune suppression, the project will determine whether novel agents in clinical trials in this Program have impact on the host immune system and are therefore likely to worsen immune funcfion. Work will also be performed to assess the nature of T cell mediated anfi-tumor immune responses against CLL cells and to determine if these specific T cell responses occur following allogeneic stem cell transplantafion for CLL. The goal here is to characterize the nature ofthe graft versus leukemia effect in CLL. To address these issues this project will address the following specific aims: First, to define the molecular mechanism whereby molecules expressed by CLL cells induce dysfunction in T and NK cell in patients with CLL and the role of immunomodulatory drug intervenfion to repair these defects. The goal here is to improve immune funcfion in CLL pafients. Second, to assess the impact of in prevention of induction of T cell defects vivo in the Emu-TCLI transgenic mouse model of CLL and asses its impact on diease progression. Third, to characterize targets of graft versus leukemia effect in CLL after allogeneic stem cell transplantafion in CTN/ CALGB 100701. Taken together, these studies will assess the impact of immune mediated responses in CLL.